The present invention relates to radiotelephone systems such as cellular phone systems, indoor cordless systems and wireless in the local loop (WLL) systems, and more particularly to methods of allocating channel parameters such as frequency, timeslots, polarization and power level in order to optimize signal quality.
The Digital European Cordless Telephone system (DECT) is an existing example of a wireless telephone system employing adaptive allocation frequency channels and timeslots for connecting a call. In the DECT systems, single-frequency duplex operation is employed by alternately transmitting a TDMA burst from a base station to an outstation and from the outstation to the base station. This is known as Time Division Duplex (TDD). The use of TDD and a common frequency for both directions of communications means that both the base and station and the outstation experience a common interference environment, particularly in indoor, wireless PABX applications for which DECT was designed. Thus, either the base station or the outstation can choose a frequency channel and a timeslot having momentarily minimum interference levels with near certainty that the chosen channel will be a good channel for communicating in both directions. In DECT, the outstation is allowed to choose the frequency and timeslot without a prior warning to the base station. The base station listens on all frequencies and timeslots in order to ensure that it always receives the signal. Signal bursts from different outstations are identified by means of a short ID code so that the base station can assemble bursts received from the same outstation on different channels. The base station transmits to the outstation using a timeslot in the transmit half of its TDD frame period corresponding to the timeslot in which the immediately previous data was received from the same outstation. The outstation listens for the base station on the receive timeslot in the receive half of its TDD frame corresponding to the transmit timeslot it used immediately previously to transmit to the base station. In this way, fast adaptation to changing interference scenarios is achieved in DECT.
The DECT system also employs adaptive antenna selection (space selection diversity) in order to mitigate slow fading caused by a wireless telephone user moving inside a building at walking pace, for example.
The base station may transmit a timeslot using a first or a second antenna, spaced so that fading of the path from the first antenna to the outstation is uncorrelated with fading from the second antenna to the outstation. The antenna used for transmitting a slot is indicated by a data bit contained in the slot. The outstation receives slots intended for it as well as slots intended for other outstations and determines whether it can receive a slot transmitted by one antenna better than a slot transmitted by another antenna. The outstation then selects a channel frequency and timeslot containing the lowest measured signal level for use in transmitting to the base station. This indicates that the channel is not in use nearby and transmits data to the base station in that slot including an indication of the base station antenna it prefers for receiving a reply.
When different frequencies are used for transmission and reception, correlation between their interference environments cannot be assumed. The polarization to be used by outstations and base stations is not defined in the DECT systems. Thus, a potential doubling of system capacity by polarization reuse is not available.
U.S. Pat. No. 5,491,837 issued to Haartsen describes adaptive channel allocation methods for use in a two-frequency duplex system. The outstations measure signals received on channels in a first frequency band from various base stations and transmit the signal measurements to the serving base station. The serving base station knows the power transmitted by all base stations and can, therefore, determine the path loss from every base station to the outstation in the first frequency band. The serving base station also knows the signals that are transmitted and the channels on which these signals are transmitted by each of the base stations and can, therefore, compute the interference scenario at the outstation on every channel, including channels not measured by the outstation. The base station also measures interference levels on channels in a second frequency band used for the communications from the outstation to the base station. The base station then determines a channel in the first frequency band for transmitting to the outstation and combines it with a channel in the second frequency band for receiving from the outstation. This combination results in good signal quality in both directions. Haartsen, however, is not concerned with determining a best polarization to use for serving a given outstation.
U.S. Pat. No. 5,548,813 issued to Charas et al. and U.S. Pat. No. 5,619,503 issued to Dent also describe the use of multiple-beam antennas for cellular and satellite communications systems. The subject matter of these documents is hereby incorporated by reference. These documents assumed that the communication path length was long so that various non-free-space propagation effects could arise to distort signal polarization. Therefore, it was difficult to count on frequency re-use with different polarization to increase capacity for communicating with mobile terminals. Polarization reuse is, however, disclosed in the context of communicating between a satellite and a fixed ground station. This type of polarization reuse is non-adaptive and both polarizations are used in the same directional beam. The ""666 patent also describes cellular base stations in which the polarization is alternated between adjacent sectors of a multiple-sector directional antenna, the main purpose of which is to obtain uncorrelated fading of the same signals received at two adjacent antennas.
It is an object of the present invention to overcome the deficiencies described above by providing a method for increasing system capacity. This is achieved by utilizing alternating polarization for short range communication, e.g., on the order of one mile, in order to increase capacity of the system by allowing frequency reuse for different signals with different polarization in adjacent sectors.
A radio base station comprises a number of directional sector antennas for providing communications with outstations lying at different azimuth angles to the base station. The directional antennas may transmit signals using one or both of two orthogonal polarizations, such as left or right hand circular polarization. A channel allocation unit allocates spectral resources, sector antennas and polarization for communicating with each outstation in a manner designed to minimize interference between different communications signals. Allocating spectral resources can include allocating a channel frequency, a timeslot or a spread-spectrum access code.
Each outstation comprises at least one antenna to receive at least one polarization, but preferably an antenna of selectable polarization. The outstation antenna may be a directional antenna, in which case it is oriented to provide maximum directional gain towards a base station that is selected to provide radiocommunication service. Each outstation comprises receiver means for receiving control channel signals from the selected base station indicative of incoming call alerts and transmitter means for transmitting responses to call alerts or call initiation requests. The outstation receiver is equipped to measure interference or signal quality levels on different frequency channels or timeslots using selected polarizations and the transmitter is equipped to transmit interference or signal quality measurements made by the receiver to the base station.
The base station is also equipped to make measurements of interference levels on different frequency channels or timeslots using selected polarizations, and to provide these measurements to the channel allocation unit along with measurements received from the outstation. These measurements enable the channel allocation unit to allocate an optimum channel and polarization for connecting a call.